One of the frontiers of photoresists involves the need for greater resolution, that is, for the resolution of smaller and smaller line widths. The reason is that semiconductor devices using such resists are packing more and more devices onto the same real estate. This is done primarily by reducing the dimensions of the devices--hence the need for smaller line widths.
The reduction in line widths has proceeded to 1 .mu.m or less, both in the line width and in the spacing. One result of this phenomenon has been the gradual abandoning by the industry of negative-working light-sensitive resists, that is, those having decreased solubility upon exposure to light, in favor of positive-working resists. Unlike the case with positive-working resists, the negative-working light-sensitive resists have a great tendency to swell upon development. Swelling is not a significant problem for line widths and spacings greater than 2 .mu.m. But at 1 .mu.m spacing or less, swelling of most negative-working resists has been enough to bridge the 1 .mu.m spacing, resulting in stringers and other unacceptable defects noticeable upon drying.
What then has been needed prior to this invention is a negative-working light-sensitive resist that has no observable swelling, during development, when the image has 1 .mu.m widths and spaces. The fact that positive-working resists will do this is not completely adequate--the availability of a negative-working light-sensitive resist would greatly expand the possible chemistries and more closely fit the processing format of some circuit designers.
Compounds have been known prior to this invention for use in resists to generate color upon exposure, as a print-out indication of exposure dosage and uniformity. Some of these, for instance nitrile-substituted aminotriphenyl methane dyes, have been found to lose a --CN moiety upon exposure, as explained in "Dosimetry for Lithographic Applications", J. Vac. Sci, Technol., Vol. 19, No. 4, Nov./Dec. 1981, p. 1343-1347. However, such compounds have not been identified, prior to this invention, as providing differential solubility. Thus, they have not been recognized as the basis for providing sensitivity to the resist, and most importantly, have not been appreciated as the basis for a negative-working resist free from swelling. Therefore, when used in resists to produce 1 .mu.m line widths or spacing, the prejudice in the trade against negative-working resists has limited such compounds to use in positive-working resists.